The present invention relates to a method of preparing a metal matrix composite material with a textured compound. In particular, it relates to a method of preparing a textured superconducting composite wire.
Some important advanced materials have layered structures. For example, high temperature oxide superconductors and the superconducting intermetallic compound MgB2 are materials with a layered structure. The layered structure causes the anisotropy in physical properties, and texture formation is necessary to achieve superior performance. As the layered compounds are brittle, the layered compounds are often prepared as metal matrix composites to provide better mechanical properties.
One well-known method to prepare such composite materials is the xe2x80x9cpowder-in-tubexe2x80x9d method. This method has been used to prepare the low temperature superconducting compound Nb3Sn, the high temperature oxide superconductors, and the recently discovered intermetallic compound MgB2. According to the formation of the compound in the powder-in-tube method, the method is also classified into two categories: the in situ technique and the ex situ technique. For the in situ technique, the components in the elemental forms are used as the starting powder, and the compound is formed inside the tube after mechanical deformation of the composite wire. For the ex situ technique, a powder of the compound is used as the starting powder.
For the high temperature oxide superconductors, both metal precursor and oxide precursor were tested, and the best results so far were achieved with the Bi2223 and Bi2212 prepared by the oxide powder-in-tube method (See, for example, a summary by D. Larbalestier, xe2x80x9cThe road to conductors of high temperature superconductors: 10 years do make a differencexe2x80x9d, IEEE Trans. Appl. Supercond. 7(2) 1997, p90-97. Also refer to a review by H. Kitaguchi and H. Kumakura, xe2x80x9cAdvance in Bi-based high Tc Superconducting Tapes and Wiresxe2x80x9d, MRS BULLETIN, February 2001, p121-125). This is attributed to the weakly-bonded, double Bixe2x80x94O layer in the structures. The easy cleavage between the Bixe2x80x94O layers introduces texturing of the phase in the deformation process. Attempts to make superconducting wires for the rare earth 123 compounds have not been very successful in achieving a high critical current density for the powder-in-tube methods. For a recent effort in this area, refer to U.S. Pat. No. 6,202,287 by A. Otto which describes an attempt to make a bi-axially aligned 123 wire from a metallic precursor powder Although the melt-texture growth method has produced high critical current densities in bulk RBaCuO-123 superconductors, the very slow growth rate is not practical for the production of long length wires. Current efforts for the rare earth 123 compounds have been focused on the coated thin-film conductors (See a review by Finnemore et al., Physica C, 320, 1999, 1). Other related developments are discussed in the following references: S. Annavarapu et al., xe2x80x9cProgress Towards a Low-cost Coated Conductor Technologyxe2x80x9d, Physica C, 341-348, 2000, p2319-2322; L. R. Motowildlo et al., xe2x80x9cRecent Progress in High-temperature Superconductors at Intermagnetics General Corporationxe2x80x9d, Physica C, 335, 2000, p44-50; R. L. Meng et al., xe2x80x9cTape processing of HBCCO, BSCCO, and YBCO Thick films on Metallic Substrates with High Jc by the Spray/Compress Techniquexe2x80x9d, Physica C, 341-348, 2000, p2315-2318; M. Zhou et al., xe2x80x9cProperties of YBa2Cu3Ox films on textured Ag tapesxe2x80x9d, Physica C, 337, 2000, p101-105; S. P. Athur et al., xe2x80x9cMelt-processing of Yb123 tapesxe2x80x9d, Physica C, 341-348, 2000, p2421-2424.
Besides the powder-in-tube method, there are many other methods for the preparation of a metal matrix composite. Thin film methods have been used to prepare all types of superconducting materials mentioned above either through physical deposition or chemical deposition. Dip coating has been used to form a layer of compound on a metal substrate. The present invention is related to a bulk method using a powder as the starting material for a layered compound.
The challenges in preparing a composite with a textured compound by a powder method include:
(a) The flow compatibility of the metal and the powder. Poor compatibility will cause difficulty in the deformation process and the formation of sausage in the composite.
(b) Powder packing. A high powder density before sintering is preferred.
(c) Densification. The decrease in density of Bi2223 during heat treatment seems to be mainly caused by the growth of non-aligned oxide grains. International Application Publication WO 01/22436A1 by Li et al., entitled xe2x80x9cSimultaneous constraint and phase conversion processing of oxide superconductorsxe2x80x9d, and the references cited disclose various ways to apply pressure during various stages of heat treatment to deal with the desintering problem to certain degree of success.
(d) Texture formation. It is desirable to develop a high degree of texture in the layered compound. For certain superconducting materials with grain boundary weak link problems, good grain boundary connectivity is also required. In the prior art, texture in Bi2212 is formed through a melt-texture growth method.
Texture in Bi2223 depends on the rolling deformation and the easy cleavage of the 2212 phase. Therefore, a high critical current density is obtained only in the tape form for Bi2223. However, round wire or wires with a low aspect ratio are more desirable for many applications.
A processing method should provide solutions to all the challenges simultaneously in order to be commercially viable. Although superconductors, and especially superconducting wires, are used as examples in the following discussion, the present invention should be applicable to other metal matrix composites with textured compounds.
Therefore, an object of the present invention is to provide a method of preparing metal matrix composite with a textured compound, which has better flow compatibility, higher packing density, better densification and texture formation. As a result, improved performance of the compound will be obtained.
Another object of the present invention is to provide a method of preparing composite wires of layered superconducting materials with a better processing condition and improved superconducting properties such as a high critical current density.
Still another object of the present invention is to provide a method of preparing composite wires of layered superconducting materials with a low aspect ratio.
Still another object of the present invention is to provide a method of preparing bulk layered superconducting materials with improved superconducting properties.
According to the present invention, the starting powder for preparing a layered compound should comprise of
(a) A plate-like powder of the layered compound or an intermediate phase (metastable phase) for the layered compound which retains its shape at least at the initial sintering stage. This plate-like powder serves as the template for texture formation of the layered compound. The xe2x80x9cplate-like powderxe2x80x9d, as the term used in this specification, has a phase with a layered structure and the particles have a minimum basal plane dimension that is at least 1.62 times greater than the thickness dimension. The minimum basal plane dimension is the shortest line segment on the basal plane through its geometry center, which is the diameter of a circular shape, the side length of a square, or the length of the shorter side of a rectangular. Because of the natural variation in powder preparation, it should be understood that the majority ( greater than 50% by volume) of the particles should meet the requirement, and preferably 80%, and more preferably 90% of the particles should meet the requirement.
(b) The remaining powder with a particle size smaller than half the median minimum basal plane dimension of the plate-like powder. Preferably this powder has a near-sphere shape, or the dimensions in different directions are similar. This second powder may contain any phases which in combination with the first powder and under certain heat treatment will form the desired final compound. Suitable phases include, but are not limited to, pure metals, alloys, intermediate compounds, and the final compound.
In the prior art, two types of powder are used (FIG. 1A and FIG. 2A). The powder shown in FIG. 1A has predominantly equal-axial or near-sphere particles. This may correspond to a well-ground uniform fine powder. This type of powder has good deformation behavior, and therefore has relatively good flow compatibility with the metal sheath. However, the powder has a low packing density and a low tendency for texture formation (FIG. 1B). The powder shown in FIG. 2A has a predominantly plate shaped particles. This powder has poor flow compatibility with the metal sheath and a low packing density (FIG. 2B). Although it has a good tendency for texture formation during the deformation process, the low packing density makes sintering difficult, and the poor deformation behavior makes the wire prone to sausage formation. Although real powders have some deviations, the powder structures in FIGS. 1A-B and FIGS. 2A-B are representative of the predominant features of the powders in the prior art for the preparation of metal matrix composites.
A well-known technique to improve powder packing density in ceramics is illustrated in FIGS. 3A-B. The basic principle is to fill the voids of the previous close packed structure with spheres of suitable sizes. The process can be repeated to several levels of voids and a high packing density of around 95% theoretical density may be obtained. When this powder is used to make a metal matrix composite wire, good flow compatibility and a high packing density would be predicted. For a compound with an easy cleavage tendency such as Bi-based oxide superconductors, a high level of texture formation can be obtained in the rolling process to form a composite tape. However, desintering may occur during the sintering process partially due to the growth of certain misaligned grains. For a compound that will not cleave easily during the rolling process, texture formation is very limited.
The proposed powder structure according to the present invention is shown in FIGS. 4A-B. The raw powder is shown in FIG. 4A, and the structure shown in FIG. 4B may represent the powder at a later stage of deformation when the initially randomly distributed plate-like particles are aligned in the deformation process. Owing to the unique xe2x80x9croller-skatexe2x80x9d structure of the powder, a good deformation behavior and a high packing density are both obtained. A high packing density means that the relative density is at least 70%, more preferably 80%, and most preferably 90%. The plate-like particles, acting like the xe2x80x9cboardxe2x80x9d of a xe2x80x9croller-skatexe2x80x9d, can slide easily on the smaller particles acting as xe2x80x9crollersxe2x80x9d. The roller particles are preferably near-sphere in shape. When the roller particles are much smaller in size (say less than 10% of the size of plate-like particles), the shape factor becomes less important. The xe2x80x9croller-skatexe2x80x9d powder structure artificially mimics the structures of the Bi2212 and Bi2223 phase where the easy cleavage between the weakly bonded Bixe2x80x94O layers provides the self-alignment property during deformation. As the attraction between the xe2x80x9cboardxe2x80x9d particles and the xe2x80x9crollerxe2x80x9d particles is even weaker in the proposed xe2x80x9croller-skatexe2x80x9d powder structure, it would be expected that texture formation will occur at a less shear strain level than the value for texture formation in the Bi-based compounds. During sintering, the plate-like particles act as templates for textured growth of the final compound. Moreover, the formation of textured grains between the plate-like particles provides an opportunity to structurally adjust possible mismatches between the plate-like particles. For oxide superconducting materials sensitive to grain boundary mismatch, this provides an extra mechanism for strongly coupled grain boundaries and hence an improved critical current density. Therefore, the present invention provides a generic method for the preparation of a metal matrix composite with a textured compound.
The volume fractions of the two powders can be estimated from a simplified model. Suppose the plate-like particle has a thickness t1 and the second powder has a packing density xcfx812 and a particle size of d2. Imagine that we can cut the powder into vertical slices and within each slice we can rearrange the plate-like particles so that the plate-like particle can form complete layers with the second powder between them. If the number of second powder particles in between is n, then the volume fraction f1 of the plate-like powder is:
f1=t1/(t1+nd2xcfx812)=1/(1nd2xcfx812/t1)xe2x80x83xe2x80x83Equation (1) 
The choice of the volume fraction of the plate-like powder is a complex issue. A suitable value should provide an adequate amount of texture templates without any significant decrease in the powder flowability. Equation (1) can serve as a general guideline for the volume fraction of the plate-like powder. Assume a typical value for xcfx812 as 74% and the particle size of the second powder equals the thickness of the first powder, we can calculate some typical values as shown in Table 1. Similar calculations can be performed for other conditions, such as the values in Table 2. An n value of about 2 or greater can be used. A larger n value will be used for a relatively smaller particle size of the second powder. For example, the f1 value of 93% in Table 2 may be difficult since bridging would occur for the first powder at this high volume fraction. The upper limit will be around the possible packing density of the plate-like powder alone. The volume fraction of the first powder would be in the range of 10 to 80% for most powders.
The choices of the particle size and shape of the powders are important considerations. For the plate-like powder, the dimension ratio between basal plane and thickness is preferably greater than 3. The particle size of the plate-like powder (measured on the average dimension of the basal plane) should be in the range of microns to tens of microns. The particle size of the second powder is preferably less than 20% of the particle size of the first powder, or in the range of microns and even smaller size. The particle size of the second powder can be controlled by grinding.
The components for the second powder can be prepared separately and then mixed together, or they can be prepared in a single powder batch. The second powder may contain particles of different size as shown in FIGS. 3A-B to obtain a high packing density.
In another aspect of the invention, the second powder further comprises at least one deformable phase so that it will facilitate the deformation of the whole powder. By deformable it is meant that the phase will either deform plastically without destruction or the particles break up into smaller pieces under the deformation process.
In still another aspect of the invention, the second powder further comprises a component or components that form a transient liquid phase during the sintering process. Such liquid forming phases include low melting components, eutectic liquid phase, and specially added liquid forming metal salts. This liquid phase will help the sintering process, and more importantly provide a mechanism for grain boundary adjustment so that low energy special grain boundaries will be formed. This will be of particular importance to many oxide superconductors.
In a preferred embodiment, the second powder comprises at least one metallic or alloy component so that a mixed mode of in situ and ex situ mechanisms is provided. This will take advantage of the merits of both mechanisms. For example, for oxide superconductors, the first powder is a plate-like oxide superconductor, and the second powder comprises the metallic powders of appropriate composition. The two powders are mixed to form a starting powder. Then the starting powder is used to make a composite wire according to the deformation process of the xe2x80x9cpowder-in-tubexe2x80x9d process to develop texture in the plate-like particles. The metallic powder is converted into oxides during the sintering process. As the plate-like particles will act as templates for the formation of the intended final oxide superconductor, highly textured superconductor grains will be formed with a high critical current density. The amount of the metallic powder in the second powder can also be adjusted as needed.
Similarly, for MgB2, the first powder is plate-like MgB2 particles, and the second powder contains Mg and B particles. B can even exist in an alloy or compound and the extra element in the alloy or compound can diffuse into the sheath material or simply exist as second phases.
In another preferred embodiment, the process to make a composite is the powder-in-tube method. A composite wire is prepared by filling the starting powder into a metal tube and reducing the cross section of the tube through mechanical deformation such as swaging, rolling, extrusion, or drawing, and for a multifilamentary wire, assembling the previously formed bundles and further deforming the assembly into a multifilamentary wire. The plate-like powder is textured during the deformation process. After sintering, improved properties are obtained due to the template texture formation.
A composite can also be prepared by other known composite processing methods, such as dip coating, tape casting, spray coating, liquid metal infiltration, and multi-layer layout.
In still another aspect of the invention, the powder is pre-textured before the major reduction process of the composite. The pre-texture operation can be accomplished by any mechanical means or magnetic alignment due to the anisotropy of magnetic properties of the plate-like powder. When a pre-textured powder is used, high reduction rolling is not necessary to achieve a high degree of texture. Therefore, instead of preparing composites in the tape form, composite wires with low aspect ratios can be prepared by more uniform deformation methods such as drawing and extrusion. The pre-textured powder also allows for the use of a variety of composite preparation methods without deformation. The pre-texture powder precursor can also be used to prepare highly textured bulk compound with improved physical properties.
By xe2x80x9ctexturexe2x80x9d as the term is used herein, it is meant that the layered compound grains have been aligned with the basal plane to a significant degree. If the phases of the second powder are different from the first powder, the texture of the plate-like powder can be easily quantified using the lotgering factor, or f-factor from the X-ray diffraction pattern of the material with a value of zero for random orientation and a value of 1 for perfect alignment. In preferred embodiments, the f-factor has a value of at least 0.6, more preferably 0.8, and most preferably 0.9. If the phase of the first powder is also included in the second powder, the texture information of the first powder may be difficult to determine due to contribution of the second powder to the X-ray diffraction pattern. If the orientation of the second powder is random, the texture information of the first powder can be obtained by certain calculation.
Otherwise, direct observation of the microstructure can provide information about the alignment. In preferred embodiments, at least 60% of the plate-like particles are within 10 degrees of the intended alignment, and this number should reach more preferably 80% and most preferably 90%.
In a preferred embodiment, the metal matrix composite is in the form of a wire. By xe2x80x9cwirexe2x80x9d as the term is used herein, it is meant an elongated article with its length dimension significantly (at least 2 times and normally orders of magnitude) larger than the dimensions of the cross section. It is equivalent to a tape, a ribbon, a rod, or the like, used in the literature.
In preferred embodiments, the layered compound is a layered superconducting material. Intermetallic compound MgB2, rare earth RBaCuO-123 oxide superconductor (where R is an element or mixture of elements of Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), Bi-based (Bi,Pb)SrCaCuO-2223 and (Bi,Pb) SrCaCuO-2212 oxide superconductors, TI-based oxide superconductors, and Hg-based oxide superconductors, are all layered superconducting materials.
In a preferred embodiment for preparing layered superconductors, the layered superconducting material is pre-textured for the preparation of the superconducting material. This allows for the use of a variety of composite preparation methods with more uniform deformation process and even no deformation process at all. For example, low aspect ratio superconducting wires can be prepared from textured feeding bar using the drawing process. FIGS. 5A-C show some of the low aspect ratio configurations.
An advantage of the powder structure of the present invention for the preparation of a layered oxide superconducting composite is that inexpensive metal substrate materials can be used instead of silver-based materials. A metallic material can be coated with an oxygen diffusion barrier layer such as a metal oxide and used as the substrate for the superconducting materials. Such metallic materials include carbon steel, stainless steel, superalloys, nickel-based materials, copper-based materials, and titanium-based materials. Since the texture formation mechanism is independent of the substrate material, more choice of substrate materials and barrier layer preparation methods are available, which in turn expands the temperature ranges and related processing methods. In addition, the superconducting layer can be made much thicker than what can be achieved in the coated superconductor method.